1. Field of the Invention
The invention relates to a process for preparing lattices stabilized by polyvinyl alcohol, the polymer powder compositions produced therefrom, and their use in hydraulically setting systems.
2. Background Art
Polymers based on vinyl esters, vinyl chloride, (meth)acrylate monomers, styrene, butadiene and ethylene are used first and foremost in the form of their aqueous dispersions or water-redispersible polymer powders in many applications. For example such polymers are used as coatings or adhesives for a wide variety of substrates. These polymers are stabilized using protective colloids or more rarely low molecular weight surface-active compounds. Polyvinyl alcohols are generally used as protective colloids. Products formed from such polymers are used, in particular, as binders in hydraulically setting adhesives such as tile adhesives based on cements or gypsum plaster.
Dispersion powders are produced, for example, by spray drying or freeze drying of aqueous lattices. The starting materials used are dispersions which are prepared by emulsion polymerization. Typically in these processes, polyvinyl alcohols are used for stabilization. The emulsion polymerization liberates a considerable quantity of heat that has to be removed at the reactor wall via the aqueous medium and then passed to an adjacent cooling medium. Emulsion polymerization has better heat transfer behavior than bulk or solution polymerizations. Nevertheless, due to the tremendous quantity of energy, the production is subject to time restrictions in order not to allow the polymerization temperature to rise above the prescribed value as a result of insufficient heat removal. Temperatures above 100° C. are usually not intended, since such temperatures can only occur in pressure-rated reactors because of the boiling point of water. In addition, the polymerization temperature has a considerable influence on the macromolecular properties of the product and thus also on the use properties. In most cases, these properties are adversely affected by the decreasing molecular weights as the temperature increases.
DE 103 35 958 A1 describes a process for preparing polymer lattices that start the polymerization at a temperature Ts and ends it at a temperature Te. Utilization of oil-soluble initiators is essential to the invention. The dispersions described are usually emulsifier-stabilized, which is a form that is not suitable for producing dispersion powders.